Chan is often referred to as the "gateless gate."
A "gate" is both a method of practice and a path to liberation. The gateway into Chan will open naturally as long as a practitioner can let go of his/her self-centered conscious mind. In response to people's needs, past Chan masters adapted other forms of practice and invented methods that made Chan more accessible.
Beginners in Chan practice use basic techniques of concentration to calm and unify the mind.
The purpose of concentration techniques is to take the mind away from a state of scattered thoughts and feelings of affliction and fetteredness. One's mind first enters a state of concentration and then enters a state where the separation between external and internal disappears. There are two major methods: the methods of huatou (of the Linji lineage, or Rinzai in Japanese), and the method of silent illumination (of the Cao Lineage, or Soto in Japanese).
A huatou is a question that you "investigate" (can). Hua means "word," and tou means "head" or "source." When practicing huatou, one wants to know, what exists before the application of any verbal or symbolic description (hua). Examples are "Who am I?", "What is wu?" (wu means "no" or "nothing"), "Who is reciting the Buddha's name?," and "Who is dragging this corpse around?" At the first level, you simply repeat the huatou. Next, you really want to know the answer, and gradually enter the stage of "one-mind." At the last stage, the one-mind breaks apart and no-mind is revealed.
The method of silent illumination (mozhao) is simply to do away with all methods of practice. Using no method is the method. Counting the breath is used when the mind is very scattered in order to enhance concentration. When you use silent illumination, your mind simply doesn't have any thoughts. You let go of everything and realize the state of Chan itself. While there are no thoughts, the mind is still very clear, very aware. Both the silence (mo) and the illumination (zhao) must be present. In this state, the mind is transparent.